This invention relates to supporting glass sheets and specifically refers to an improved construction of tongs for gripping thin glass sheets during thermal treatment. Tongs have been used for supporting glass sheets in an essentially vertical plane for thermal treatment by gripping the opposite glass sheet surfaces near their upper edges. The tongs are supported from an overhead rail which extends through thermal treatment apparatus by means of a supporting clevis mounted to a carriage that rides on the rail.
During thermal treatment, the glass sheets are heated as uniformly as possible to an elevated temperature required for subsequent treatment. In tempering, the subsequent treatment involves uniform quenching of the heat-softened glass sheets as uniformly as possible. Glass gripping tongs have been provided with glass engaging elements as small as possible in order to minimize the localized obstruction to the flow of the quenching fluid.
Since glass sheets are heated to substantially the softening point during thermal treatment required for tempering or for certain coating procedures, and since tongs for gripping glass sheets are provided with glass engaging elements that penetrate the heat-softened glass sheet surfaces and mar the latter, particularly when the glass sheets are relatively thin so that they require more intensive heating to arrive at a subsequent treatment station at a temperature suitable for the subsequent treatment, a need has existed for tongs with glass engaging elements that do not penetrate the opposite surfaces of the glass sheet as deeply as the prior art tongs.
Disc-shaped glass engaging elements have been used successfully on glass gripping tongs for thermal treatment of glass sheets of previous commercial thicknesses (at least 4.8 millimeter) with reduced tong marking and penetration. However, the prior art tongs needed larger diameter discs to grip thinner glass sheets properly than the discs provided previously to grip glass sheets of greater thickness. The larger sized discs interfered with the flow of quenching fluid against the glass sheet surfaces in the vicinity of the disc-shaped glass engaging elements. This interference in free flow of quenching fluid caused lower compression stresses in the glass sheets in the vicinity of the glass engaging elements than elsewhere in the cooled glass, thus leading to weak regions.
Glass sheets are usually shaped between a pair of complementary pressing members that require clearances for the glass gripping tongs during the shaping operation. The thinner the glass sheet undergoing shaping, the more critical are the requirements for maximum size of notches or openings in the glass sheet shaping members. When the clearance notches normally provided in press bending molds are too large, the control of the shape of the glass sheets is lost in the vicinity of the notches. This produces glass sheets that do not conform to the specifications established by the customer. Glass sheets that deviate from curvature to a great extent are difficult to install.